The invention relates to a device for measuring filling levels, particularly for determining the filling height of a filling material in a container.
Measurement of the filling level in general involves the use of measuring systems that determine the distance between a sensor and the filling material on the basis of the measured transit time of electromagnetic waves traveling from a sensor mounted in the container lid to the surface of the filling material and back. The filling level can then be calculated from a knowledge of the container height.
Sensors of this type are known under the technical designation of filling level radar and are based overall on the property of allowing electromagnetic waves to propagate within a homogenous, non-conductive medium at a constant speed and to be reflected at least partially at the border area of different media. Every border layer between two media with different dielectric constants produces a radar echo upon impact of the wave. The greater the difference in the two dielectric constants, the greater the change in the wave resistance of wave propagation and the greater the observed echo.
Different radar methods are known for determining the wave transit time. The two primary methods are the pulse transit time method (pulse radar method) and the frequency-modulated continuous wave radar method=FMCW radar method). The pulse radar process makes use of pulsing amplitude-modulation of the transmitted wave and determines the direct period of time between transmission and reception of the pulses. In the FMCW radar process the transit time is determined indirectly by transmitting a frequency-modulated signal and determining the difference between the transmitted and the received momentary frequency. Other process are described, e.g., in DE 44 04 745 C2.
In addition to the different radar principles, various frequency ranges of the electromagnetic waves are used, depending on the application. For example, there are pulse radars with carrier frequencies in the range from 5 and 30 GHz, as well as some which work in the base band as so-called monopulse radars without a carrier frequency. Also known is a series of different methods which guide the electromagnetic wave to the surface of the filling material and back.
A fundamental distinction is made between waves emitted into space and waves that are guided by a conductor. Radar sensors which guide the electromagnetic wave to the reflecting point and back by means of wiring are often referred to as TDR (time domain reflectometry; in German, direct current pulse reflectometry) sensors. As compared to sensors which freely radiate high-frequency waves, these sensors exhibit a considerably lower attenuation in the reflected echo signal, since the power flow occurs only in a narrowly limited range in the area around the conductive waveguide. Furthermore, spurious echoes from the inside of the container are largely avoided with sensors of this kind. Such spurious echoes can arise from reflections of the wave on components installed in the container (e.g., stirring devices, tubes, etc.), and in the case of freely radiating sensors they can make it more difficult to identify an echo signal. As a result, level measurement that is performed with conducted electromagnetic waves is largely independent of the container construction and also of the product characteristics of the filling material and of other operating conditions (e.g., dust, angle of bulk material, etc.), and thus provides very reliable measuring results.
Suitable as a waveguide for conducting the wave is any high-frequency electrical line in which the wave at least partially penetrates the medium that the metallic conductor surrounds or that is enclosed by said metallic conductor. Specifically, parallel-wire lines, single-wire lines, coaxial lines, and high-frequency waveguides have proven to be particularly suitable in this connection. The metal parts used here for the sensors can either be produced from corrosion-proof steel or provided with an insulating layer for protection against aggressive filling materials. A level sensor with a guided wave is described by way of example in DE 44 04 745 C2.
The measuring range of a sensor is generally limited by the length of the waveguide, which is usually referred to as a probe in devices of this kind for determining the level of filling material. Since, as a rule, the electric transmitting and receiving device is mounted in the container lid, the probe runs downward from the mount opening in the lid and ends above the container floor. To also allow filling levels to be measured when the container is only slightly full, it is expedient for the probe to end as close to the container floor as possible. If, when the container is only slightly full, the end of the probe is immersed just a few centimeters into the filling material, the resultxe2x80x94as expectedxe2x80x94is a reflection at the border area of the filling material; but since this reflection is not a total reflection of the entire wave, a larger or smaller portion of the wave will penetrate the filling material, depending on the size of the dielectric constants of the filling material. This penetrating portion meets the end of the probe, which usually represents an electrical open circuit. On the other hand, there is an electrical short circuit in the case of a likewise only partially used, uninsulated dead-ending of the probe against the container floor (a cable probe may be mentioned here by way of example). In both cases the probe end totally reflects the arriving wave. An additional echo is thereby produced.
The pulse width of an echo is dependent on the modulation bandwidth of the modulated transmission signal. In pulse radar, for example, this is dependent on the pulse width of the transmitted pulse. In the case of a container that is only slightly filled, if echoes are produced from two closely neighboring reflection points (for example, from the probe end and from the surface of the filling material), these echoes may partly overlap in time. For customary pulse widths of about 10 to 30 cm, which correspond to modulation bandwidths of about 0.75 to 1.75 GH, the two echoes overlap until the end of the probe is immersed in the filling material by an amount that corresponds to the pulse width. In the evaluation process it is difficult to separate these overlapping echo portions from reflections at the surface of the filling material and reflections at the end of the probe, and a problem arises in that the sought-after position of the filling material cannot be precisely determined.
When the filling level is low the evaluation of the echo is difficult and an exact determination of the filling level is almost impossible, particularly in the case of filling materials for which the dielectric constant exhibits lower values and for which, therefore, the amplitude of the filling material echo is small as compared to the amplitude of the echo from the probe end.
Known from EP 0 780 665 A2 is a filling level sensor for which spurious echoes for an empty container are measured and stored. Spurious echoes are eliminated by subtracting the empty signal from the actually measured echo signal. However, this method is not suitable for eliminating the influence of the spurious echoes from the probe end, since at the beginning of the filling this echo changes in its temporal sequence relative to the echo of the filling level surface due to the reduced propagation speed of the wave within the filling material, and is furthermore reduced in amplitude by dampening within the filling material. The elimination of an unwanted signal by means of subtraction is only successful when an exact assignment according to time and amplitude is possible, and this is ruled out in the case of spurious echoes from the end of the probe, for the indicated reasons.
The goal of the present invention is to propose a filling level measuring device that correctly measures filling levels even at a low level of fill or at a slight immersion depth on the part of the waveguide.
This goal is solved by a filling level device having a waveguide having a wave absorption device for at least partial absorption of the conducted electromagnetic waves. Advantageous embodiments and elaborations of the invention are also described.
The basic idea of the invention consists in providing the end that is immersed in the filling material with a wave absorption device which at least partly absorbs the introduced electromagnetic waves. This assures that an electromagnetic wave running to the end of the probe can only be reflected to a limited degree at this point, thus providing an effective reduction in spurious echoes.
The type of wave-absorption device depends on the type of design and construction of the waveguide (probe) itself, as the following exemplary embodiments will make clear.
In all conventional probesxe2x80x94particularly single-wire, double-wire, coaxial, and waveguide probesxe2x80x94absorption can be provided by applying a wave-dampening material to the end of the probe. In high-frequency technology, various materials are available to achieve this end, materials which can be selected according to the different frequency ranges and the required degrees of dampening.
As a preferred embodiment, reference is made to a material that has a fine distribution of conductive pigments within a filling material.
Devices of this kind can be easily produced. It is only necessary to assure that a corresponding body formed from this material can be applied to the end of the waveguide. The design is not restricted in advance and can be determined by criteria such as ease of production and/or mechanical expediency.
It proved to be particularly advantageous to use carbon black particles as conductive pigments. Pigments of this kind are attractive in terms of cost and are available in practically any desired thickness and size.
Silicon is employed as the preferred filling material, since it can also be cheaply obtained and permits any desired shape. Furthermore, it is inert as compared to a number of liquids that come into consideration as filling materials.
In a particularly advantageous embodiment of the invention, the wave-dampening material is designed as a cone-shaped body. This assures that the dampening effect increases or decreases over the exact position on the end of the waveguide.
Another embodiment of the invention provides that the wave-dampening material exhibits a dampening gradient. This is achieved by selecting a shape in accordance with the desired dampening behavior and by choosing the concentration of the pigments in the filling material accordingly, or by using pigments of differing conductivity.
In the case of two-wire and coaxial probes it is possible as an alternative to insert an ohmic resistor between the two ends of the line, at the probe end. For this ohmic resistor it is preferable to select a resistance value that lies in the vicinity of the wave resistance of the probe (waveguide).
It is particularly advantageous to choose a resistance value that is somewhat smaller than the resistance possessed by the probe when it has not yet been immersed in the filling material. This guarantees that when the container is completely empty a small, usable echo arises at the probe endxe2x80x94an echo that can be used for displaying the empty status of the container. If the probe end is immersed in the filling material, the wave resistance of the immersed portion of the probe is reduced, due to the dielectric constants of the filling material. An ideal selection of the terminal resistor will assure that this resistance coincides with the wave resistance of the immersed probe, and the wave will be completely absorbed.
As a terminating resistor, it is advantageous to have an SMD resistor that exhibits low inductance and is capable of high-frequency operation. Resistor wires or wired resistors are also possible.